Receptor and cytoplasmic tyrosine kinases trigger signaling through common pathways in which lie other critical regulators of cell growth. Together, these signal transducers govern the response to extracellular mitogens which drive progression through the G1 phase of the cell cycle. Critical rate limiting regulators of G1 progression include MYC, the D-and E-type G1 cyclins, and their catalytic cyclin-dependent kinase (CDK) partners. NIH3T3 fibroblasts expressing a mutant human colony-stimulating factor-1 receptor [CSF-1R (Y809F)] exhibit attenuated receptor kinase activity in response to CSF-1 and fail to proliferate when stimulated with the growth factor. These CSF-1 stimulated cells remain viable and undergo induction of various immediate early response genes, such as FOS and Jun family members, but they do not manifest MYC, cyclin D or E induction, and they remain arrested in early G1 phase. Rescue of the receptor defect has identified common signal transducers, including MYC itself, ETS factors, and the D-type cyclins, that act as essential downstream components required for S phase entry. We will utilize an unbiased approach to identify essential components of signaling pathways that functionally connect CSF-1 induced mitogenic signals to MYC and cyclin D function. Cells expressing CSF-1R[Y809F] will be infected with retroviruses encoding mammalian cDNA libraries, and those cDNAs that specify proteins able to conditionally rescue mitogenicity in response to CSF- 1 will be identified. Selected genes will be studied for their ability to induce MYC expression or to bypass its functions. Many positive regulators of G1 progression (including CSF-1R, MYC, and cyclin D1) can function as collaborating oncogenes when mutated or constitutively overexpressed, whereas their opposing negative regulators function as tumor suppressors. We therefore anticipate that newly identified transducers of mitogenic signals identified here will similarly be implicated in oncogenesis when genetically altered or inappropriately expressed. The identification of novel effector molecules should enable definition of regulatory networks that govern events late in G1 phase, including the commitment to enter S phase.